Preparation of Catalyst Precursors by Metal Impregnation onto Catalyst supports using various impregnation techniques, is well known to those skilled in the art. The impregnated supports so obtained are then usually subjected to drying and calcination to provide catalyst precursors, and the precursors are then subjected to reduction to produce, finally, a catalyst.
In particular, the Applicants are aware that, as described in EP 0736326 B1, cobalt impregnated alumina based Fischer-Tropsch synthesis catalysts can, for example, be synthesized by means of aqueous slurry phase impregnation of a cobalt salt, for example cobalt nitrate hexahydrate, onto an alumina support, coupled with drying of the impregnated support, followed by direct fluidized bed calcination of the resultant impregnated support, to obtain a catalyst precursor, and then reducing the precursor to obtain the Fischer-Tropsch synthesis catalysts. The catalysts contain cobalt dispersed on the support. Sufficiently high cobalt loadings to provide a desired high degree of catalyst activity can readily be obtained by means of the cobalt salt impregnation, by repeating, if necessary, the cobalt salt impregnation step.
It has been reported that the use of organic metal compounds or organic additives during the impregnation could assist in increasing the catalyst activity of supported metal catalysts. For example, U.S. Pat. No. 5,856,260 teaches that using mixtures of polyols and metal salts during impregnation results in improved catalyst performance.
Van de Loosdrecht et. al. (Applied Catalysis A: General, Volume 150, Number 2, 13 Mar. 1997, pp 365-376(12)) reported that the preparation of low loading cobalt catalysts (2.5% Co) by impregnation using Co-EDTA (ethylenediaminetetraacetic acid) or ammonium cobalt citrate resulted initially in the formation of very small cobalt oxide particles. These small oxide particles reacted during thermal treatment in a reducing gas flow with the alumina support to form cobalt aluminate, which was inactive in Fischer-Tropsch synthesis. Higher loading catalysts (5% Co) prepared by a 2 step impregnation process using ammonium cobalt citrate in both steps resulted in a larger cobalt oxide particle size and higher reducibility, culminating in a reasonable activity, but still lower compared to a reference catalyst prepared from impregnation with cobalt nitrate only and having similar cobalt loading.
The use of organic impregnation compounds tends to result in low metal loadings due to limited solubility and high viscosity of the impregnation solution. For many catalytic reactions, the low loadings of metal do not provide sufficiently high activity, due to, amongst other reasons, the low reducibility of such catalysts.
Kraum and Baern (Applied Catalysis A: General 186 (1999)189-200) describe studies of the performance of titania supported catalysts containing 12% cobalt, prepared by multiple impregnations with various organic cobalt compounds, including cobalt(III) acetyl acetonate, cobalt acetate, cobalt oxalate and cobalt-EDTA. The multiple impregnations were performed for each particular catalyst using only a single organic cobalt compound. In this study, sufficiently high metal loadings were only achieved by performing multiple impregnation cycles, thereby compensating for the low solubility of the organic cobalt compounds. However, having to use multiple impregnation cycles for the preparation of a catalyst can be economically unattractive.
U.S. Pat. No. 6,822,008B2 teaches the use of two different metal precursors loaded separately on to a suitable support, in such manner that the first loaded portion of metal is more easily reduced, than the second loaded portion of metal. For example, cobalt nitrate hexahydrate is loaded first, whereafter cobalt acetate is loaded on to the support. The second cobalt precursor can thus be an organic cobalt salt such as cobalt acetate. However, U.S. Pat. No. 6,822,008B2 does not demonstrate an increase in cobalt dispersion or an increase in FT synthesis activity.
It is thus an object of the present invention to provide a catalyst precursor and/or a catalyst with which some of the disadvantages described above are overcome or at least reduced.